Material for treating acidic waste water

ABSTRACT

This invention relates to a method for treating acidic waste water, particularly mine effluent, and to a solid waste water treating material useful for the method. This waste water treating material is obtained by solidifying a mixture of rock wool and an inorganic binder mainly containing at least one kind selected from silicates, hydroxides and oxides of alkaline earth metals and alkali metals and has a porosity of 50% or more. When brought into contact with acidic waste water containing iron ions and sulfate ions, this waste treating material can not only neutralize the waste water but also remove harmful heavy metals such as iron and arsenic. Furthermore, it is easy to dispose the spent waste water treating material.

FIELD OF TECHNOLOGY

This invention relates to solid waste water treating materials and amethod for treating acidic waste water and, in particular, it relates toa method for neutralizing acids in mine effluent to remove heavy metalssuch as iron and arsenic.

BACKGROUND TECHNOLOGY

Waste water from acidic hot springs in volcanic regions, acidic mineeffluent and acidic underground water in regions of volcanic soilcontain sulfuric acid formed by the oxidation of sulfur-containingsubstances and iron sulfide ores. such acidic waste water exerts anadverse influence on concrete structures such as bridges and dams and,still more, acids and heavy metals such as iron and arsenic thereincontained degrade water quality, exterminate fish and shellfishes andcause the so-called “red river” phenomenon when dischargedindiscriminately. For this reason, it is necessary to submit acidicwaste water to a neutralization treatment.

A method in wide use for this neutralization treatment consists ofadding particles or a slurry of slaked lime to waste water. The chemicalsubstance used in this method is relatively low in cost and has anexcellent ability to neutralize acidic waste water; however, in thecases where waste water contains sulfate ions and iron ions in largequantities, the iron ions precipitate taking the form of colloidalferric hydroxide as the pH rises and, besides, the sulfate ions reactwith slaked lime to form difficultly soluble gypsum and precipitatetogether with some of the unreacted slaked lime to form a slimy matterthat is high in water content and difficult to dewater. While this ishappening, heavy metals such as arsenic in waste water are adsorbed onthe ferric hydroxide and precipitate together. As this slime is a highlyhydrous slurry that is difficult to dewater and contains harmfulsubstances, its disposal requires installation of a solid-liquidseparator such as an expensive thickener, a sedimentation basin and adevice for dewatering and compacting slime such as a labor-consumingfilter press and construction of a dam for accumulating slime as a finaldisposal device. Thus, the disposal of slime poses problems of increaseddisposal cost and harmful influences on natural environment.

In order to reduce formation of slime that is highly hydrous anddifficult to dewater, the use of magnesium oxide particles as aneutralizing material has been studied because magnesium oxide formsslime that is easier to dewater and does not form difficultly solublereaction products such as gypsum, but a high cost of the chemical inquestion is a disadvantage.

Furthermore, in order to reduce cost and improve the dewateringperformance of slime, the use of calcium carbonate particles andlimestone grains has been tried. However, gypsum resulting fromneutralization covers the surface of calcium carbonate or limestone tohinder a further progress of neutralization thereby reducing theefficiency in utilization of the neutralizing material. A neutralizingmaterial based on calcium carbonate produces only a small effect forraising the pH and cannot precipitate ferrous ions in waste water asferrous hydroxide. Therefore, a pretreatment becomes necessary tooxidize ferrous ions to ferric ions by aeration or by iron-oxidizingbacteria.

Application of inorganic fibers to a treatment of waste water as afiltering material or a material for binding microorganisms is disclosedin JP6-315681A and elsewhere but nothing is taught of the use ofinorganic fibers as a material for neutralizing acidic waste water.

JP2000-73347A discloses drainage materials composed of inorganic fibersand inorganic hydraulic materials for underdrainage, but they areregarded as substitutes for the husks of rice used up to the present.

DISCLOSURE OF THE INVENTION

Accordingly, an object of this invention is to neutralize acidic wastewater and remove harmful heavy metals such as iron and arsenic. Anotherobject is to provide a solid waste water treating material which canprevent an occurrence of “red water” in rivers, is suited for serviceover a long period of time and exhibits a distinct ability to removeheavy metals. A further object is to provide a method for treating wastewater which requires no costly equipment such as a neutralizationdevice, thickener and press nor manpower and can be practicedpractically without need of motive power and a source of electricity andwithout requiring maintenance. A still further object is to provide amethod for treating acidic waste water which prevents useless sulfateions from entering the spent treating material after execution of wastewater treatment and facilitates waste disposal by reduction of volumeand water content.

The solid waste water treating material of this invention is obtained bysolidifying a mixture of rock wool and an inorganic binder mainlycontaining at least one kind selected from silicates, hydroxides andoxides of alkaline earth metals and alkali metals and exhibits aporosity of 50% or more.

Further, this invention relates to a method for treating waste watercontaining iron ions or sulfate ions or both and comprises bringing theaforementioned solid waste water treating material into contact withsaid waste water thereby removing 80% or more of the iron ions orneutralizing acidic waste water exhibiting a pH of 5 or less to a pH of6 to 8.

The solid waste water treating material of this invention (hereinafteralso referred to as waste water treating material) is obtained bysolidifying mineral fibers with an inorganic binder.

Mineral fibers containing silicates of alkaline earth metals or alkalimetals are used here. Mineral fibers preferably contain 30–50 wt % ofSiO₂, 5–20 wt % of Al₂O₃, 30–50 wt % of MgO and CaO, 0–10 wt % of Na₂Oand K₂ O and 0–10 wt % of others. Rock wool and slag wool are examplesof such mineral fibers and rock wool is preferred because of itsdistinct ability to neutralize acidic waste water.

This neutralizing ability is adequate when 10 g of the material is addedto 1000 ml of an acidic solution containing 2500 mg/l of sulfate ionsand 370 mg/l of Fe⁺² ions and exhibiting a pH of 1.8 and allowed toreact with the stirred solution at room temperature for 24 hours therebyrendering the pH of the solution to 3 to 6, preferably 4 to 5, after thereaction. When the pH of the solution is lower than 3 after thereaction, the reactivity in neutralization is low and acids may remainin the treated water. On the other hand, when the pH of the solution ishigher than 6 after the reaction, the reactivity in neutralization istoo high and the components of rock wool constituting the waste watertreating material leach into the water being treated and, as a result,the rock wool loses its property as fiber and deteriorates in waterpermeability and dewaterability.

Rock wool is produced by melting a variety of slags such as blastfurnace slag and electric furnace slag, natural rocks such as basalt anddiabase or a mixture thereof in an electric furnace or cupola andfiberizing the molten mass by a centripetal force or a pressurized gas.Rock wool contains CaO, SiO₂ and Al₂O₃ as main components and furthercontains MgO, Fe₂O₃ and others. A typical composition is 35–45 wt % ofSiO₂, 10–20 wt % of Al₂O₃, 0.1–3 wt % of Fe₂O₃, 4–8 wt % of MgO, 30–40wt % of CaO and 1–4 wt % of MnO. Rock wool can readily be processed intogranular products, exhibits excellent water permeability and waterretention, contains dead air spaces suitable for growth ofmicroorganisms and has a function to neutralize acidic waste water onaccount of its basic chemical composition.

Rock wool to be used in this invention may be virgin rock wool, wasterock wool containing 50 wt % or more of rock wool or recovered rockwool.

Virgin rock wool is available in several shapes such as layered rockwool and granular rock wool and granular rock wool is preferred. Layeredrock wool is processed into granular rock wool by a granulator or arotary screen and a material with an average particle diameter in therange of 1–50 mm, preferably in the range of 5–40 mm, is suitable. Also,use may be made of granules obtained by cutting or crushing a moldedrock wool article which is obtained by adding recovered rock wool and abinder followed by molding into a board or the like.

An inorganic binder useful for solidifying rock wool is mainly composedof at least one kind of silicates, hydroxides and oxides of alkalineearth metals and alkali metals, typically Ca, Mg, Na and K. Preferredinorganic binders are one or two kinds or more of cement, water glass,slaked lime, quicklime, magnesia, slag particles and fly ash and theyare preferably hydraulic and possess a function to neutralize acids. Ahydraulic inorganic binder is allowed to harden in the presence ofwater.

The acidic waste water neutralizing ability of a treating material ispreferably such that 10 g of the material added to 1000 ml of an acidicsolution containing 2500 mg/l of sulfate ions and 370 mg/l of Fe⁺² ionsand exhibiting a pH of 1.8 is allowed to react with the stirred solutionat room temperature for 24 hours thereby rendering the pH of thesolution to 6 or more. When the pH of the solution after the reaction isshort of 6, both inorganic binder and rock wool leach out simultaneouslyduring the neutralization reaction and the waste water treating materialloses the property as fiber and deteriorates in water permeability anddewaterability.

Hydraulic inorganic binders of the aforementioned kind include cement(typically portland cement), mixtures of latent hydraulic substancessuch as blast furnace slag particles and alkali materials and slakedlime which reacts with mineral fibers such as rock wool to causesolidification thereof. Other types of cement are available besidesportland cement; for example, blast furnace slag cement, fly ash cement,magnesia cement, alumina cement and lime-mixed cement. Portland cementor blast furnace slag cement is preferable.

The mix ratio of rock wool and an inorganic binder varies with the kindof inorganic binder and an inorganic binder is normally used in aproportion of 10–60 wt %, preferably 20–50 wt %, of the sum total ofrock wool and inorganic binder. Excessive use of an inorganic binderreduces porosity and deteriorates water permeability.

The method for mixing rock wool and an inorganic binder is notrestricted and a known mixer such as ribbon blender can be used for thispurpose. In case an inorganic binder is hydraulic, water is added in arequired amount to cause solidification at the time of mixing or at thesite of use after the mixture is transported there or after the mixtureis applied there. It is further allowable to mix additionally a materialreactive with acids such as limestone particles as occasion demands.

There is no restriction on the shape of the waste water treatingmaterial of this invention and one of preferred shapes is granular. Tomanufacture a waste water treating material which is molded andsolidified in a granular shape, rock wool, an inorganic binder and waterare mixed in a known mixer such as a ribbon blender and rotarygranulator, molded into granules and solidified. The use of granularwool such as granular rock wool is advantageous in that the granulatingoperation can be omitted. In the case of granules, the average particlediameter is 1–200 mm, preferably 5–50 mm.

Another of preferred shapes is a sprayed structure. The sprayingtechnique used for applying fireproof coating to buildings can beadopted here. This technique performs mixing of granular rock wool,cement and water and spraying at the same time and a mixture of cementand rock wool, a mixture of cement and water or a mixture of rockwooland water may be prepared in advance. In order to provide a layer of thewaste water treating material of this invention by the sprayingtechnique, granular rock wool as mineral fiber and cement as aninorganic binder are sprayed together with water and allowed to solidifyto an average thickness of 5–300 mm, preferably 10–100 mm.

Another advantageous method comprises preparing a mixture of rock wooland a hydraulic inorganic binder in advance and solidifying the mixtureby contact with water to give a solid waste water treating material.

A still another advantageous method comprises of filling a containerwith a mixture of rock wool and an inorganic binder and solidifying themixture by contact with water to give a solid waste water treatingmaterial. This method becomes more advantageous if this container is areactor for effecting the contact with water.

It is necessary for the waste water treating material of this inventionto have a porosity of 50% or more regardless of its shape. The porosityis preferably in the range of 70–98%. Moreover, the bulk specificgravity of waste water treating materials is set in the range of0.1–1.5, preferably in the range of 0.15–1.0.

The bulk specific gravity and porosity can be determined by knownmethods. The porosity is determined by weighing a cube of 1 cm³ cut froma waste water treating material when it is dry (A g) and again when itis completely impregnated with water (B g) and computing the differenceB−A.

Concretely, the porosity is determined in accordance with the method fordetermining the porosity in three phase distribution of soil (solidphase, liquid phase, and gas phase).

With the aid of a commercial soil sampler for a soil three phase meter(available from Daiki Rika Kogyo Co., Ltd.), a sample is gently cut outof the waste water treating material in the shape of a cylinder with adiameter of 50 mm and a height of 51 mm at the application site. In casethe thickness of the layer of waste water treating material is short of51 mm, a required number of layers are added one over another until thethickness reaches or exceeds 51 mm and a sample is cut out. The cuttingoperation is performed in compliance with the directions for handlingthe soil sampler.

The sample cut out of the waste water treating material in this manneris mounted on a commercial soil three phase meter (actual volume of soil(solid phase+liquid phase)) and an apparatus for measuring the gas phaseaccording to the Boyle's law (available from Daiki Rika Kogyo Co., Ltd.)and the actual volume (solid phase+liquid phase) and the weight (wet) ofthe sample are determined in accordance with the procedure for operatingthe apparatuses.

To determine the proportion occupied by the liquid phase in the actualvolume, the sample is dried sufficiently at 110

and weighed and the weight (dry) was subtracted from the weight (wet) togive a water content.

Since the internal volume of the sample is 100 ml, the porosity (%) canbe calculated from the numerical values obtained above as follows;Porosity (%)=100−(measured actual volume)+(water content)

The bulk specific gravity is calculated by dividing the weight (dry) bythe internal volume of the sample or 100 ml.

When the porosity is too high or the bulk specific gravity is too low,the amount of waste water treating material per unit volume becomesinsufficient and so becomes the neutralization treatment in some cases.When the porosity is too low or the bulk specific gravity is too high, asufficient contact is not attained between acidic waste water and thetreating material.

The waste water treating material of this invention is applicable toacidic waste water of any kind and is particularly effective fortreating acidic waste water that contains iron ions or sulfate ions orboth and exhibits a pH of 5 or less, preferably 1 to 4.

The aforementioned solid waste water treating material is useful as awaste water treating material in the method for treating acidic wastewater of this invention and the method is particularly effective fortreating waste water that contains iron ions, preferably Fe⁺² ions, andsulfate ions and exhibits a pH of 5 or less, preferably 1 to 4.

Although there is no restriction on the kind of waste water to betreated by the method of this invention, mine effluent is treatedadvantageously. Mine effluent is waste water discharged from a mine andcontains both sulfate ions formed by the oxidation of sulfur and ferrousions. Mine effluent oozes out of passageways to form a small stream,small streams unite to form a large stream which flows out of thepassageways and mines or accumulates in a depression to be pumped out.Mine effluent is collected in storage tanks and ponds, treated anddischarged into rivers.

The waste water treating material is also useful for acidic mineeffluent oozing or flowing out of heaps of waste stones containing ores,outcrops of ores, abandoned mining sites such as open-pit mines and slagheaps of a smelting works.

Acidic waste water, particularly mine effluent, amenable to a treatmentby the treating material of this invention is the one that shows an 8.3acidity (the amount of alkali consumed to neutralize to pH 8.3) or a 4.8acidity of 300 mg-CaCO₃/l or more and an iron ion concentration of 30ppm or more and it is possible to reduce the 8.3 acidity to 200mg-CaCO₃/l or less, the 4.8 acidity to 100 mg-CaCO₃/l or less and theiron ion concentration to 10 ppm or less by the treatment performed inaccordance with this invention. That is, the pH rises less and the ironion concentration decreases more compared with ordinary neutralizingmaterials.

Acids present in mine effluent and in underground water in regionscovered by volcanic mud are mostly sulfuric acid while acids in hotsprings in volcanic regions are mostly sulfuric acid and hydrochloricacid. The iron ion concentration in mine effluent is normally in therange of 50–500 ppm and mine effluent containing iron ions in an amountexceeding this range can be dealt with by applying a larger amount ofthe treating material.

In the cases where the waste water treating material of this inventionis provided by spraying in a layered structure, it is preferable tobuild a sprayed structure at the sites where mine effluent oozes out orwhere oozed effluent forms a small stream. In such a case, the wastewater treating material is applied by spraying at the site of treatmentwhere waste water oozes out, for example, at the mouth of a mine, heapsof waste stones containing ores, outcrops of ores, abandoned miningsites such as open-pit mines and slag heaps of a smelting works or thetreating material is applied to the whole area covering heaps andabandoned sites. At the aforementioned sites where mine effluent flowsat a low rate, a layer of waste water treating material of even amoderate thickness can maintain a longer contact time. Rainwater thathas passed through the waste water treating material of this inventionshows a pH in the range of 8–12 because of leaching of alkali metals oralkaline earth metals contained in the treating material and thisenhanced alkalinity is effective for reducing the activity ofsulfur-oxidizing bacteria and iron-oxidizing bacteria and retarding theoxidation of sulfides contained in ores and slags thereby giving promiseof reduced generation of acidic water.

At the site where mine effluent forms a large stream, the waste watertreating material of this invention is advantageously applied byproviding layers filled with granular waste water treating material atsuch sites and passing the mine effluent through the layers. In thiscase, it is preferable to control the thickness of filled layers or theflow rate in such a manner as to obtain a contact time of 30 minutes ormore, preferably 1–5 hours, between the effluent and the waste watertreating material. Moreover, the treated effluent is controlled at a pHof 6 to 8, preferably 6.5 to 7.5.

In the cases where the waste water treating material of this inventionis used at the sites where mine effluent is first stored in a storagetank or a pond, a granular waste water treating material is added as itis to the mine effluent or it is packed in a basket-like container andsubmerged or suspended in the mine effluent. In the cases where thespent waste water treating material is recovered and replaced with a newmaterial, the use of a container offers an advantage.

It is further possible to fill a treating tank with a waste watertreating material and pass acidic waste water through the tank. In thiscase, an adequate procedure is to pass acidic waste water from the topthrough the tank filled with a granular waste water treating material,collect the effluent in a receiving trough provided beneath the tank anddischarge as treated water. It is adequate here to control the thicknessof the layers of waste treating material in the range of 100–2000 mm andthe contact time in the range of 0.5–5 hours.

The use of a combination of two methods or more described above may beadvantageous and these methods can be applied to acidic waste waterother than mine effluent.

The temperature at which the treating material is kept in contact withwaste water is satisfactorily room temperature and the contact time is30 minutes or more, preferably 60 minutes or more, although it varieswith the amount of filled material, throughput of waste water andconcentration of the substances to be treated in waste water.

In a method for treating waste water containing iron ions or sulfateions or both, a preferable procedure is to solidify a mixture of rockwool and an inorganic binder by contact with water to give a solid wastewater treating material having a porosity of 50% or more and bring thissolid waste water treating material into contact with waste watercontaining iron ions to remove 80% or more of iron. In this case, theconcentration of iron ions in the waste water to be treated ispreferably in the range of 100–250 ppm.

Moreover, with the use of the solid waste water treating material ofthis invention, iron ions precipitate when waste water having a pH of 3or less is neutralized to a pH of 4 to 6 by contact with the treatingmaterial and removal of iron is preferably effected under thiscondition.

In another method of treating waste water containing iron ions orsulfate ions or both, it is advantageous to solidify a mixture of rockwool and an inorganic binder by contact with water to give a solid wastewater treating material and bring this treating material into contactwith acidic waste water having a pH of 5 or less thereby effectingneutralization to a pH of 6 to 8.

When the waste water treating material of this invention is brought intocontact with acidic waste water, the alkaline earth metals and alkalimetals in the treating material react with acids and silicic acidremains as amorphous silica. Sulfate ions partly react with calcium inthe waste water treating material to produce gypsum, but the amount ofgypsum thus produced is small because of the presence of other alkalineearth metals and alkali metals. In consequence, sulfate ions mostly formharmless water-soluble sulfates and are discharged. Iron ions in mineeffluent are mostly ferrous ions and, when brought into contact with thewaste water treating material of this invention, their reaction with thematerial progresses slowly and in the meantime ferrous ions are oxidizedby dissolved oxygen and the like to ferric ions and precipitate asferric hydroxide. Mine effluent sometimes contains heavy metals such asarsenic and cadmium and most of these metals can be precipitated andremoved by contact with the waste water treating material of thisinvention.

When the waste water treating material of this invention is used,alkaline earth metals and alkali metals decrease and ferric ions formedin a large amount in the reaction precipitate as ferric hydroxide and itis desirable to replace or add the treating material immediately beforethe pH of the treated water or an indicator of the ability as an acidicwaste water treating material drops below the value specified for thetreating site or immediately before the iron ion concentration in thetreated water reaches 10 mg/l.

Replacement or addition of the treating material is preferably performedas follows. When the treating material stops giving the specifiedperformance in waste water treatment, the treating material is renewedas follows; rock wool and an inorganic binder are sprayed again with orwithout removal of the spent material, a mixture of rock wool and aninorganic binder is added or the spent mixture is replaced with a newlyprepared mixture or the container is refilled with a freshly preparedmixture. In this case, it is preferable to follow the renewal procedurewhile adding water to solidify the waste water treating material and theaddition of water may be made either after or simultaneously with mixingof rock wool and the inorganic binder. In case the container is to befilled with the mixture, it is advantageous to add water to solidify themixture of rock wool and inorganic binder immediately after filling.

The spent waste water treating material contains the unreactedsilicates, reaction residues mainly containing silica, and reactionproducts comprising a large mount of iron compounds and a small amountof gypsum. Therefore, the spent waste water treating material can beused as an iron-containing soil conditioner and the like and is easilydisposed unless the material in question contains harmful componentssuch as arsenic. It is preferable for the solid waste water treatingmaterial of this invention to contain 50 wt % or more of amorphoussilica in the residual treating material after use in waste watertreatment.

The rock wool-containing waste water treating material of this inventioncan prevent the treated water from becoming excessively alkaline anddoes not require readjustment of the pH by acid. Moreover, silicate gelsformed from rock wool coprecipitate with iron colloids to directlyreplace rock wool and form a solid fibrous aggregate and, as a result,slime which is hard to dewater does not form and at the same timeprecipitation of iron contained in waste water is accelerated.Furthermore, since rock wool contains other readily leachable cations,the neutralization reaction is hindered very little by gypsum. Stillfurther, the dewatering performance does not deteriorate very much ifthe treating material is rendered granular for higher waterpermeability. When the products in the spent treating material containlittle harmful heavy metals, the spent material can be utilized as asoil conditioner and the like.

PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1

Granular rock wool (granular S-FIBER, average particle diameter 30 mm,available from Nippon Steel Chemical Rockwool Co., Ltd.) was used asrock wool.

To begin with, the test for leaching of rock wool was performed. Rockwool was pulverized finely in a mortar, 1 g of the pulverized rock woolwas immersed in 150 ml each of pure water, 2% aqueous citric acid, 0.25N hydrochloric acid and 0.5N hydrochloric acid and the amounts ofleached alkaline earth metals, alkali metals, silica and alumina weredetermined (expressed in ppm of the components leached from 1 g of rockwool). The results are shown in Table 1. The analysis was carried out inaccordance with the method for analysis of fertilizers.

It is seen from Table 1 that rock wool reacts not only with hydrochloricacid but also with a weak acid such as citric acid.

After this, a granular acidic waste water treating material (averageparticle diameter 20 mm, porosity 95%) obtained by solidifying 100 g ofa mixture of 60 wt % of rock wool and 40 wt % of blast furnace slagcement with 100 g of water was pulverized finely in a mortar, thepulverized treating material was added to 1000 ml of acidic effluentthat was sampled at a mine of iron sulfide ores, contained 1300 mg/l ofsulfate ions and 135 mg/l of total iron ions and exhibited a pH of 2.8,batch tests were carried out to determine the changes in pH with timeand the results are shown in Table 2. Removal of total iron ionsdetermined 1 hour after the addition was 99.9%. All the tests werecarried out at room temperature with stirring.

These results indicate that rock wool by itself cannot remove iron ions,but rock wool converted into a waste water treating material inaccordance with this invention can remove iron ions as the iron ionspenetrate into the material and participate in the reaction.

EXAMPLE 2

A pulverized molded rock wool article (rock wool for plant cultivation,available from Grodan Co., average particle diameter 50 mm, porosity92%) was used as rock wool and it was finely pulverized in a mortar andtested for leaching as in Example 1 to give the results shown in Table1.

After this, the aforementioned rock wool was substituted for the rockwool in Example 1 and a mixture of the aforementioned rock wool andblast furnace slag cement was solidified to form a waste water treatingmaterial (average particle diameter 50 mm, porosity 95%), pulverizedfinely in a mortar as in Example 1 and tested for the changes in pH togive the results shown in Table 2. Removal of total iron ions was 99.2%when determined 1 hour after the addition.

TABLE 1 Na K Ca Mg Si Al Example 1 Pure water 11 15 230 17 27 4 0.2%Citric acid 720 2000 190000 24000 69000 31000 0.25 N HCl 730 1900 17000024000 43000 30000 0.5 N HCl 730 2000 200000 24000 59000 31000 Example 2Pure water 16 3 22 17 16 10 0.2% Citric acid 17000 26000 76000 35000120000 54000 0.25 N HCl 17000 25000 75000 33000 110000 54000 0.5 N HCl21000 29000 90000 39000 120000 64000

EXAMPLE 3

The same rock wool as used in Example 1 was mixed with portland cementin a ribbon blender in a weight ratio of 60 of the former to 40 of thelatter, the mixture was sprayed with water weighing the same as themixture, and left standing overnight to give a granular solid (a wastewater treating material) exhibiting an average particle diameter of 20mm and a bulk specific gravity of 0.196.

This waste water treating material (100 g) was added to 1000 ml of thesame waste water as used in Example 1 and batch tests were carried outto determine the changes in pH to give the results shown in Table 2.Removal of total iron ions determined one hour after the addition was98.0%.

TABLE 2 Time elapsed Example 1 Example 2 Example 3 4 6.2 6.1 6.4 8 6.36.2 6.9 16 6.5 6.6 6.9 24 6.6 6.9 6.9 72 6.9 7.2 7.1

EXAMPLE 4

The same rock wool as used in Example 1 was mixed with portland cementin a ribbon blender in a weight ratio of 64 of the former to 36 of thelatter to give an unsolidified waste water treating material exhibitingan average particle diameter of 20 mm and a bulk specific gravity of0.17.

Then, a container measuring 90 cm in height, 120 cm in length and 16 cmin width provided with a plastic net at the bottom was filled with 20 kgof the unsolidified waste water treating material to give a 60 cm-thickmass with a porosity of 92% and a bulk specific gravity of 0.20 and thismass was solidified by adding water of the same weight as the mass togive a solid waste water treating material.

Upon completion of solidification, 50 m³ of acidic mine effluent havingthe water quality shown in Table 3 was passed from the top of thecontainer at a rate of 14.5 L/hr. Addition of the neutralizing materialper 1 m³ of the mine effluent was 0.4 kg/m³ during this treatment.

The treated water flowing out of the bottom of the container showed ironremoval of 99.9% and arsenic removal of 94.2%. The iron content in thespent waste water treating material was 53% and 14% of the neutralizingmaterial remained. The water quality of the treated water is shown inTable 3.

The water permeability of the waste water treating material was 1.0×10⁻²cm/s initially and 0.6×10⁻² cm/s after passage of 50 t of mine effluent.The volume of the waste water treating material was 88% of the initialvolume after passage of 50 t of the mine effluent. The water content inthe waste water treating material after passage of 50 t of the mineeffluent was 77.2% on the average when determined 30 minutes afterpassage of the mine effluent was stopped and the bulk specific gravityafter drying at 110° C. was 184 kg/m³.

TABLE 3 Water T—Fe As SO₄ ²⁻ 8.3 acidity 4.8 acidity quality pH mg/lmg/l mg/l mg-CaCO₃/l mg-CaCO₃/l Acidic mine 2.8 90.3 0.12 956 830 768effluent Treated 4.1 0.05 0.007 960 64 1 water

COMPARATIVE EXAMPLE 1

Reagent-grade calcium hydroxide in 325-mesh particles was added to thesame acidic mine effluent as used in Example 4 and the amount of calciumhydroxide required to neutralize the mine effluent to the same 8.3acidity as in the case of the treated water in Example 4 was determined.It was 0.58 kg/m³ per 1 m³ of the mine effluent. To 1000 ml of this mineeffluent was added 580 mg of slaked lime as a neutralizing material, themixture was stirred at room temperature for 24 hours and the entire lotwas filtered through a vacuum filter (a suction funnel, inside diameter70 mm, No. 5C filter paper). The time required for filtration was 342seconds. The water permeability of the filtered material was 4×10⁻⁶cm/s.

COMPARATIVE EXAMPLE 2

The amount of sodium hydroxide to neutralize the same acidic mineeffluent as used in Example 4 to the same 8.3 acidity as in the treatedwater obtained in Example 4 was determined by adding an aqueous sodiumhydroxide solution (1 mol/l) to the acidic mine effluent according tothe method for analysis of water quality specified by JIS and thisamount was used to calculate the amount of calcium carbonate required toneutralize the acidic mine effluent to 8.3 acidity. It was 0.77 kg/m³per 1 m³ of the mine effluent. To 1000 ml of this mine effluent wasadded reagent-grade calcium carbonate in 325-mesh particles, the mixturewas stirred at room temperature for 24 hours and checked for its pH,which was 6.9. The entire lot was then filtered through a vacuum filter(a suction funnel, inside diameter 70 mm, No. 5C filter paper). The timerequired for filtration was 93 seconds. The water permeability of thefiltered material was 1×10⁻⁵ cm/s.

COMPARATIVE EXAMPLE 3

The same acidic mine effluent as used in Example 4 was introduced at arate of 0.05 m³/min to an oxidation tank with an internal volume of 1m³, the same reagent-grade calcium carbonate in particles as used inComparative Example 2 was added at a rate of 0.20 kg/m³ per 1 m³ of themine effluent to keep the pH inside the oxidation tank at 3 to 4 andiron-oxidizing bacteria and air were blown into this reaction systemthereby oxidizing ferrous ions to ferric ions. The reaction mixture wasthen introduced to a neutralization tank with an internal volume of 1m³, the same reagent-grade calcium carbonate in particles as used inComparative Example 2 was added at a rate of 0.57 kg/m³ per 1 m³ of themine effluent, the resulting mixture was stirred to precipitate ferricions as ferric hydroxide, and the effluent containing this reactionproduct was settled in a thickener with a diameter of 2 m and aninternal volume of 5 m³ to effect separation into the treated water andthe reaction product. The reaction product was returned to theneutralization tank at a rate of 0.02 m³/min to raise the utilizationefficiency of the neutralizing material and the dewaterability of thereaction product. The slurry obtained from the thickener showed a watercontent of 99% at this time and the slurry dewatered under a pressure of11 kg/cm² by a filter press with a filtering area of 0.25 m² showed awater content of 76%.

Removal of iron from the original mine effluent was 99.9% and that ofarsenic was 99.8%, the iron content in the reaction product after usewas 38% and the rate of survival of calcium carbonate was 45%. Theconcentration of sulfate ions at that time was reduced to 74% of that ofthe original mine effluent. However, compared with Example 4, a largeramount of the neutralizing material was used per 1 m³, the iron contentin the reaction product was lower and the rate of survival of unreactedcalcium carbonate was higher than those in the examples, and moreunreacted calcium carbonate remained; this means that the treatingmaterial was utilized less efficiently and the reaction products took insulfate ions that needed not to be removed thereby gaining weight andincreasing the work load at the final disposal site. Moreover, theoperation requires extra steps of oxidation by air blowing anddewatering under pressure of the reaction product by a filter press andthis was a shortcoming in respect to labor saving and capital cost.

Industrial Applicability

The method for treating acidic waste water by the use of acidic wastewater treating material of this invention effects not onlyneutralization but also removal of harmful heavy metals such as iron andarsenic. The waste water treating material of this invention reacts withacidic waste water thereby causing more to form soluble compounds andless to remain behind, the water permeability is maintained after useand volume reduction becomes possible and a dewatering apparatus such asa filter press becomes unnecessary.

1. A method for treating acidic waste water containing iron ions, oriron ions and sulfate ions, which method comprises solidifying a mixtureof (i) granular rock wool and (ii) an inorganic binder comprising atleast one selected from the group consisting of silicates, hydroxides,and oxides of alkaline earth metals and alkali metals to form a solidwaste water treating material having a porosity of 70–98% and a bulkspecific gravity of 0.1–1.0, and bringing said solid waste watertreating material into contact with acidic waste water containing ironions, or iron ions and sulfate ions, to thereby remove 80% or more ofthe iron ions.
 2. A method for treating acidic waste water as describedin claim 1, wherein the concentration of iron ions is 100–250 ppm.
 3. Amethod for treating acidic waste water as described in claim 2, whereinthe removal of iron is effected under such neutralizing conditions as tochange the pH of said acidic waste water from 3 or less before treatmentto 4 to 6 after treatment.
 4. A method for treating acidic waste watercontaining sulfate ions, or sulfate ions and iron ions, which methodcomprises solidifying a mixture of (i) granular rock wool and (ii) aninorganic binder comprising at least one selected from the groupconsisting of silicates, hydroxides, and oxides of alkaline earth metalsand alkali metals to form a solid waste water treating material having aporosity of 70–98% and a bulk specific gravity of 0.1–1.01, and bringingsaid solid waste water treating material into contact with acidic wastewater having a pH of 5 or less to thereby effect neutralization of saidacidic waste water to a pH of 6 to
 8. 5. A method of providing a wastewater treating material for treating acidic waste water containing iron,which method comprises spraying, mixing or filling in a container (i)granular rock wool and (ii) an inorganic binder comprising at least oneselected from the group consisting of silicates, hydroxides and oxidesof alkaline earth metals and alkali metals, wherein said waste watertreating material has a porosity of 70–98% and a bulk specific gravityof 0.1–1.0. and contacting said waste water with said waste watertreating material.
 6. A method for treating acidic waste watercontaining iron, which method comprises spraying, mixing or filling in acontainer (i) granular rock wool and (ii) an inorganic binder comprisingat least one selected from the group consisting of silicates, hydroxidesand oxides of alkaline earth metals and alkali metals, to provide awaste water treating material having a porosity of 70–98% and a bulkspecific gravity of 0.1–1.0, bringing said waste water treating materialinto contact with acidic waste water containing iron, and upon adecrease in performance of said waste water treating material,reapplying said granular rock wool and said inorganic binder with orwithout removing the used waste water treating material.